Thermal imaging processes are well-known. In these processes a donor element comprising a colorant is heated, by, for example, a thermal head or an infra-red laser, causing the colorant to be transferred to a receptor sheet. Depending on the process, the colorant may be a dye or pigment or a mixture of dyes and/or pigments. Imagewise heating of the donor element reproduces the corresponding image on the receptor sheet. Transfer in register to the same receptor sheet from several differently colored donor elements produces a multicolored image. Different single colored donor elements or a multicolor donor element carrying different colors in different regions which can be brought into position in turn can be used for transfer.
When an infra-red laser is used for thermal transfer, only a single, small, selected area is heated at one time. Since only a small region of colorant is heated and transferred, the image can be built up pixel by pixel. Computer control of such processes allows multicolor images of high definition to be produced at high speed. This process is disclosed in Baldock, UK Patent 2,083,726.
In the laser-induced thermal dye transfer process, the donor element comprises a heat transferable dye, sometimes called a thermal transfer dye, usually in a formulation with a binder, supported on a substrate. The dye donor element is contacted with a receptor sheet, and the surface of the substrate irradiated with an infra-red laser to transfer the dye to the receptor sheet. For the heat transferable dye to be directly heated by the laser, a dye which strongly absorbs the wavelength of the exciting laser is required. This need to match the infra-red absorption of the dye to the emission of the laser greatly restricts the number of dyes which can be used in the laser-induced thermal transfer process.
As an alternative, the dye may be heated indirectly by incorporating a separate radiation absorber, such as carbon black, into the dye layer. However, carbon black has a tendency to aggregate or agglomerate when coated so that the absorber is not uniformly distributed in the donor element. In addition, small carbon black particles tend to be carried over with the dye, contaminating the image.
Alternatively, an infra-red absorbing compound can be added to the dye layer. Dye donor layers containing infra-red absorbing materials have been disclosed by, for example, Barlow, U.S. Pat. No. 4,778,128, which discloses thermal printing media comprising infra-red absorbing poly(substituted)phthalocyanine compounds; DeBoer, EPO Application 0 321 923, which discloses infra-red absorbing donor elements which contain cyanine dyes; and DeBoer, U.S. Pat. No. 4,942,141, which discloses infra-red absorbing donor elements which contain selected squarylium dyes. However, there is a continuing need for infra-red absorbing materials which may be used to advantage in laser-induced thermal transfer processes.